1. Field of the Invention
This invention relates generally to analog-to-digital converters and analog-to-digital converting techniques, and more particularly is directed toward the new and improved method and associated circuitry for measuring an unknown quantity such as voltage, power, etc., by means of a tridual phase analog-to-digital technique.
2. Description of the Prior Art
A great many applications exist where an analog quantity or value must be measured accurately and presented in digital form. Various types of analog-to-digital converters are available and adapted to convert an analog value in the form of voltage, power, current, resistance, speed, strength, pressure, etc., into digital form. In electronic analog-to-digital converters two basic techniques are utilized. In one class the input quantity, usually voltage or current, is converted into another form, such as a pulse duration or a frequency. The intermediate quantity is then measured to provide a digital representation of the input signal. In the other class of A/D converters, the input is directly compared with a known reference signal which can be varied under control of the analog-to-digital converter logic.
Within these two classes many techniques are available for analog-to-digital conversion. The five major techniques most often used in data acquisition and process control computers are the ramp, the integrating-ramp, voltage-to-frequency, successive-approximation, and the parallel-serial methods. The first three of these methods involve the conversion of the input signal into an intermediate quantity, which is then measured; the latter two are direct comparison methods.
In the ramp A/D converter, the input voltage and a linear ramp voltage V = Kt, where K is the slope, are compared by using a comparator amplifier. Since the slope of the ramp is known, the time duration between a zero ramp voltage and equal input and ramp voltages is a measure of the magnitude of the input signal. This time duration is measured by counting constant-frequency pulses with a counter circuit.
In the integrating ramp A/D converter, the input signal is integrated for a fixed period of time. A constant reference voltage of opposite polarity is then integrated until the output of the integrator reaches zero. Since the duration of this second integration period is proportional to the magnitude of the input signal, a digital representation is obtained by measuring the length of this period using a counter and a constant frequency pulse source.
In the voltage-to-frequency technique, the input signal controls the frequency of a variable frequency oscillator. The average frequency of the oscillator over a fixed time interval can be measured using digital methods to obtain a digital representation of the input signal.
The successive approximation A/D converter is an example of a direct comparision conversion technique. The A/D converter consists of a digital-to-analog converter which is used as a digitally controlled voltage reference source, a comparator and control logic. The conversion consists of a sequence of comparisons between the input signal and reference voltage values derived from the D/A converter. The number of comparison steps required for a binary successive approximation A/D converter is equal to the number of bits in the digital representation.
The parallel serial technique is a modification of the successive approximation method and is used for high speed A/D converters. In this technique three or four bits of the digital output representation are determined simultaneously by using multiple comparators.
Although all of these A/D converters are used in data acquisition and process control computer system, there are certain disadvantages present in each system, such as complexity, cost, accuracy, stability and reproducibility.
Accordingly, it is an object of the present invention to provide an improved method and associated circuitry for converting an analog value to a digital value on a highly reproducible, accurate and stable basis. Another object of this invention is to provide an A/D converter having a simplified analog portion for minimizing sources of potential errors.